In a mixing bowl, add 1 cup of besan (gram flour), 1/4 teaspoon of turmeric powder, 1/2 teaspoon of red chilli powder, and salt to taste. Mix well.
Gradually add water to the mixture while stirring continuously to form a smooth batter without lumps. The consistency should be similar to that of a pancake batter.
Add 1 finely chopped onion, 2 finely chopped tomatoes, 2 finely chopped green chillies, and a handful of chopped coriander leaves to the batter. Mix well.
Heat a non-stick pan on medium flame and grease it lightly with oil.
Pour a ladleful of the batter onto the pan and spread it gently to form a thin circle.
Drizzle a little oil around the edges and cook until the bottom side of the omelette is golden brown.
Flip the omelette and cook the other side until it is also golden brown.
Transfer the tomato omelette to a plate and serve hot with green chutney or ketchup.